Simplified: What are gravitational waves?

The Laser Interferometer Gravitational-Wave Observatory or LIGO finally detected gravitational waves in September 2015. Results of further analyses further confirmed the existence of this elusive natural phenomenon. Experts described this discovery as a scientific landmark akin to having the ability to hear and see for the first time. But what are gravitational waves? What is the importance of this discovery?

Brief history of gravitational waves

Albert Einstein first predicted the existence of gravitational waves when he introduced his general relativity theory in 1915. He argued that the movement of heavy objects in the universe causes a ripple in the spacetime fabric. But the German theoretical physicist struggled to elaborate this concept further.

But the theory of general relativity proved to be useful for scientists to understand gravity and other related concepts. The reliability of this theory as a framework for observing and understanding the universe compelled other scientists to confirm the existence of the proposed these waves.

Joseph Weber made a claim in 1969 that his experiments led to the detection of gravitational waves. However, the failure of others to replicate his experiments deemed his claim invalid. An indirect evidence for this phenomenon nonetheless emerged in 1974 when Joseph Taylor and Russell Hulse discovered a binary pulsar.

A more modern attempt to confirm the existence of gravitational waves started in 1979 when the United States National Science Foundation provided funding for the construction of LIGO. Construction began in 1994 and the project became operational in 2002 but it experienced several setbacks that compelled observes to upgrade LIGO to improve the sensitivity of the instruments.

The Advanced LIGO detected a signal that appeared to come from the collision of two black holes. A thorough analysis of the data revealed that the detected signals were gravitational waves. The discovery was made public in February 2016.

Simplified definition of gravitational waves

The general relativity theory claims that the gravitational field of an object changes whenever it changes its configuration. But the gravitational field cannot change instantaneously over all space because nothing can travel faster than light. The changes in the field instead spread out from the source in the form of gravitational waves.

Consider gravitational waves as similar to sound waves. A vibrating object causes a ripple in a medium such as air or sound that results in the generation of an audible mechanical wave or sound. The same is true for this natural phenomenon in the universe. Moving or accelerating objects cause ripples in the spacetime fabric.

To better illustrate gravitational waves, think of spacetime as a flat rubber sheet. Placing a bowling ball would cause a curvature in the sheet the same way planets and stars cause a curvature in the spacetime. The heavier the object, the more curved the sheet becomes. Moving this bowling ball would also move the point or position of curvature. This would cause a ripple as the rubber sheet stretches and squeezes from different directions. Dropping the bowling ball would not only cause a curvature but also a ripple because of the distortion on the sheet.

Another way to think of gravitational waves is to compare them to dropping a stone in a pond. Because of the acceleration, the stone would not only cause a curvature but also a distortion in the surface of the water. The ripples are similar to gravitational waves.

Everything with mass and energy essential can make gravitational waves. Particularly, the acceleration or change in the configuration of objects causes a ripple in the spacetime fabric. Even the movements of people and objects in the Earth cause a ripple in the fabric of space and time. But the ripples are usually negligible because they are extremely small and practically undetectable.

Detectable gravitational waves

From the aforementioned definition and illustrations, gravitational waves occur when mass accelerates or change its configuration. This acceleration causes ripples or distortions that travel across the spacetime fabric.

Ripples caused by the acceleration of an object or mass disappear quite quickly however, thus making these waves hard to detect. The strongest ripples should come from massive objects changing configurations at high velocity. In other words, most violent events produce distortions in the spacetime fabric that are massive enough to become detectable in Earth.

Example of these violent events include gravitational collapse leading to the formation of black holes, the condensation of matter before galaxy formation, a neutron star and a black hole pair, and a pair of neutron starts or black holes orbiting one another.

The gravitational waves detected by LIGO in September 2015 originated from the merging of two black holes. This merger transpired billions of years ago. It resulted in waves travelling outward the spacetime fabric and passing through matter or objects in space for 1.3 billion years.

Behind the collision of the two black holes that happened 1.3 billion years ago is a theory that both circled each other. They did this as they became closer and closer over the course of a billion year before finally colliding in a fraction of a second. The result is a single black hole. In addition, the collision resulted in the loss of energy that was released as waves or ripples.

The black holes were theorised to be about 30 times the mass of the Sun and they were moving by about half the speed of light in the last fraction of second before they collided. The impact sent a huge shockwave or ripples in the spacetime fabric of the universe—travelling at the speed of light.

Even these gravitational waves caused by the merger of two black holes and travelled to the Earth were very miniscule. The distortion in the spacetime fabric detected by LIGO was as small as a 1/1000 diameter of a proton. It is akin to identifying a hum from an environment with a loud background noise.

Importance of gravitational waves discovery

The discovery of gravitational waves is tantamount to having the ability to hear and see for the first time. Or in other words, this is akin to having a new tool for understanding and exploring the universe further.

History tells that every time scientists discover and understand a natural phenomenon, they also discover things they did not expect or confirm the claims of previous predictions from their theories. Gravitational waves help in further understanding how the universe works.

The discovery of LIGO actually provides a prime example. It confirms the predictions made by Einstein and his general relativity theory. Confirming the existence of gravitational waves also provides another strong support for the general relativity theory.

This discovery is also the first time that scientists have observed the merger of two black holes. It confirms the existence of binary black hole system.

Another importance of the discovery is that it might open new areas in physics and astronomy. For example, studying the phenomenon generated by black hole mergers or other violent events could uncover newer insights about the workings of the universe.

Gravitational waves nonetheless provide a tool for probing the universe—similar to how the discovery of X-ray has allowed physicians to probe the human body.